Heart pacers such as that described in U.S. Pat. No. 3,057,356 issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart whereby it is contracted at a desired rate in the order of 72 beats per minute. Such a heart pacemaker is capable of being implanted within the human body and operative in such an environment for long periods of time. Typically, such pacemakers are implanted in the pectorial region or in the abdominal region of the patient by a surgical procedure, whereby an incision is made in such region and the paceamer with its own internal power supply, is inserted within the patient's body. This pacer operates asynchronously to provide fixed-rate stimulation not automatically changed in accordance with the body's needs, and has proven effective in alleviating the symptoms of complete heart block. An asynchronous pacer, however, has the possible disadvantage of competing with the natural, physiological pacemaker during episodes of normal sinus condition.
An artifical pacer of the demand type has been developed wherein the artificial stimuli are initiated only when required and subsequently can be eliminated when the heart returns to the sinus rhythm. Such a demand pacer is shown in U.S. Pat. No. 3,478,746 issued Nov. 18, 1969 and entitled "CARDIAC IMPLANTABLE DEMAND PACEMAKER". The demand pacer solves the problem arising in asynchronous pacers by inhibiting itself in the presence of ventricular activity (the ventricle's R wave), but by coming "on line" and filling in missed heartbeats in the absence of ventricular activity.
A problem with such prior art, implantable demand pacers it that there was no way to temporarily increase or decrease the rate or other operating parameter at which these stimulating pulses are generated without surgical intervention. Still another problem is the great difficulty in ascertaining a failing electrode, and in establishing an adequate R-wave sensitivity safety margin in an implanted demand pacer.
Another improvement which has occured since Greatbatch first disclosed the implantable cardiac pacemaker is means to allow the pacemaker to be reprogrammed after it has been implanted. In U.S. Pat. No. 3,805,796 in the name of Reese Terry, Jr. et al, entitled "Implantable Cardiac Pacer Having Adjustable Operating Parameters", which issued in 1974, circuitry is disclosed to allow the rate of the pacemaker to be noninvasively changed after it has been implanted. The rate varies in response to the number of times a magnetically operable reed switch is closed. The Terry et al device operates by counting the number of times the reed switch is closed and storing that count in a binary counter. Each stage of the counter is connected to either engage or bypass one resistor in a serially connected resistor chain, which chain is a part of the RC time constant controlling the pacemaker rate.
The concept of the Terry et al device has been improved upon by the apparatus shown in U.S. Pat. No. 4,066,086 in the name of John M. Adams et al, entitled "Programmable Body Stimulator", which issued in 1978, and which discloses a programmable cardiac pacemaker that responds to the application of radio frequency (RF) pulse bursts while a magnetic field held in close proximity to a magnetically operated reed switch included within the pacemaker package holds the reed switch closed. In the Adams et al circuit, again only the rate is programmable in response to the number of RF pulses bursts applied. The use of radio frequency signals to program cardiac pacemakers was earlier disclosed by Wingrove in the U.S. Pat. No. 3,833,005 entitled "Compared Count Digitally Controlled Pacemaker" which issued in 1974. The Wingrove device was capable of having both the rate and pulse width programmed.
One area where cardiac pacing technology has lagged behind conventional state of electronic technology involves utilization of digital electrical circuits. One reason for this has been the high energy required to operate digital circuits. However, with more recent technology advances in complimentary metal oxide semiconductor (CMOS) devices fabricated on large scale integrated circuits, together with the improvements of cardiac pacemaker batteries, digital electronic circuits are beginning to be utilized in commercial pacemakers. The inherent advantages of ditial circuits are their accuracy, and reliability. Typically, the digital circuit is operated in response to a crystal oscillator which provides a very stable frequency over extended periods of time. There have been suggestions in the prior art for utilizing digital techniques in cardiac stimulators and pacemakers since at least 1966. For instance, see the article by Leo F. Walsh and Emil Moore, entitled "Digital Timing Unit for Programming Biological Stimulators" in The American Journal of Medical Electronics, First Quarter, 1977, pages 29 through 34. The first patent suggesting digital techniques is U.S. Pat. No. 3,557,796 in the name of John W. Keller, Jr., et al., and is entitled "Digital Counter Driven Pacer", which issued in 1971. This patent discloses an oscillator driving a binary counter. When the counter reaches a certain count, a signal is provided which causes a cardiac stimulator pulse to be provided. At the same time the counter is reset and again begins counting the oscillator pulses. Additionally, in the Keller et al. patent, there is disclosed the digital demand concept, in which the counter is reset upon the sensing of a natural heartbeat, and the digital refractory concept, in which the output is inhibited for any certain time after the provision of a cardiac stimulating pulse or the sensing of a natural beat.
As mentioned above, digital programming techniques are shown in both the Terry et al. U.S. Pat. No. 3,805,796 and the Wingrove U.S. Pat. No. 3,833,005. Wingrove additionally discloses digital control circuitry for controlling the rate of the stimulating pulses by providing a resettable counter to continually count up to a certain value that is compared against a value programmed into a storage register. The Wingrove patent also shows provisions for adjusting the output pulse width by switching the resistance in the RC circuit which controls the pulse width.
Though there has been suggested that various parameters, i.e., pulse width and rate, may be changed within an internally implanted pacer, it is desired to provide a device that is capable of operating in various, different pacing and/or sensing modes. The systems of the prior art are capable of storing by means of digital counter circuitry a programmable word indicative of desired rate or pulse width. In an internally implanted device, the space to incorporate a plurality of such counters whereby a number of such functions could be programmed, is indeed limited.
In FIG. 1, there is shown an universally programmable pacemaker system 10', which includes a programmer 12, a programming head 14, and an implantable pacemaker 16. Signals generated by the pacemaker 16 are applied through leads 18 to the heart (not shown) to cause the contraction thereof. The type of signals applied from pacemaker 16 through leads 18, as well as the response of the heart to these signals, is well known in the art and will not be discussed herein.
In the above-identified application entitled "Digital Cardiac Pacemaker", there is described a programmable pacemaker such as pacemaker 16 which includes means adapted to provide electrical stimulation signals to at least one lead for stimulating body tissue and sensing means for detecting the natural currents of electrical activity of the patient's heart and in response to such electrical activity including the patient's ECG and pacer pulses, generates and transmits corresponding electrical signals. As described in detail in the noted application, the pacemaker 16 includes a program storage means for accepting and storing program sigals to influence the electrical stimulation circuitry and the sensing circuitry. The programmer 12' is designed to transmit coded signals to such a pacing generator illustrated in FIG. 1 as element 16 to effect changes of the mode and the parameters of the stimulating mode effected upon the patient, as well as to change the manner of sensing the electrical activity of the patient's heart. The keyboard is made of up a switching array to provide a plurality of switches or switching points, whereby different parameters and modes of operation may be programmed.
Ilustratively, it is desirable to be able to program the sensitivity of the sense amplifier included within the pacemaker 16 so that it can more or less be sensitive depending on the patient's needs. Inn addition, it is desirable to be able to program the refractory period of the pacemaker 16, which is not readily apparent from viewing the EKG. Further, it is desirable to be able to program the stimulating rate, the width of the pacer pulses; the amplitude of the stimulating pulses; in a hysteresis mode of operation, the percentage less of the rate of which the pacing generator will initiate its stimulation from the limit to be detected to initiate such pacing; and a set of nominal values of these parameters at which if the operator elects, to return the operation of the pacing generator in an emergency situation. In addition, the pacemaker 16 may be programmed in a variety of modes of operation including a demand mode of operation, a synchronous operation in which the pacemaker pacing generator is made to generate pacing pulses synchronous with the detecting of the patient's R wave, asynchronous mode in which the pacing generator applies stimulating pulses at a fixed rate, an inhibit mode of operation in which the programmed operation of the pacing generator is inhibited so that the physician may observe the normal contracting of the patient's heart without the aid of the pacing generator, a measure mode of operation in which the patient's heart activity including his EKG and pacing pulses are measured and displayed, a temporary mode of operation in which a desired set of parameters are adopted for a test period and an auto-threshold mode of operation in which the pulse width of the pacing pulse applied to the patient's heart is incrementally decreased until the pacing pulses are no longer able to stimulate the patient's heart, i.e., heart capture is lost. It is noted that it is not only necessary to be able to transmit coded messages to the pacemaker 16, which may be implanted as noted above in the patient, but also to receive and to monitor the effect of the changing of these parameters by the display of the patient's heart activity. The pacemaker 16 described in the above-identified application entitled "Digital Cardiac Pacemaker," is capable of checking whether the parameter or mode of operation sought to be programmed has been actually entered into the storage means of the pacing generator. Further, provision must be made to prevent operator error so that inappropriate parameters or modes of operation may not be entered into at inappropriate times.
In the above-identified application entitled "PROGRAM TESTING APPARATUS," there is described in detail the programmer 12' for transmitting encoded signals to the pacemaker 16 to effect the programming of a mode and/or parameter of the pacemaker's operation. The programmer 12' includes electrodes coupled to the patient's body for sensing his heart activity signals including the stimulating pulses, a keyboard for entering a parameter and/or mode od operation to be programmed within the pacemaker 16, a transmitter for encoding and transmitting the parameter, and/or mode of operation to the pacing generator, and a measuring circuit for measuring the characteristics of the detected heart activity and the artifacts generated by the pacemaker 16 to provide a manifestation of the patient's heart activity and a manifestation that the pacemaker 16 has been successfully programmed. The programmer 12' includes a display for providing the manifestations to the operator of the successful programming of the pacing generator.
The programmer 12 as described in the above-identified application entitled "PROGRAM TESTING APPARATUS" was designed to program a wide variety of parameters and modes of operation, and was energized through AC outlets as would by typically found in a hospital or a Doctor's office. By contrast, the subject invention is directed toward a portable, battery-powered programmer for the pacemaker 16, as described in the above-identified patent application entitled "DIGITAL CARDIAC PACEMAKER". As indicated above, there has been a hesitancy to incorporate digital devices including micro-processors into pacemakers or any battery operated device such as the contemplated programmer, due to the high power requirements of such components. This problem is further compounded when it is realized that the encoded signals are transmitted by RF transmission to the internally implanted pacemaker, the transmitter of the programmer requiring even higher power for operation than normally would be contemplated for the other, digital components of the programmer.